Layers of superconducting cuprate materials Bi.sub.2 Sr.sub.2 Ca.sub.n-1 Cu.sub.n O.sub.y (n=2 or 3) and Tl.sub.2 Ba.sub.2 Co.sub.2 Cu.sub.3 Oy and TlB.sub.2 C.sub.2 Cu.sub.3 Oy have been formed as a thin film on Ag tape to provid superconducting tapes.
The newly discovered high-temperature superconducting (HTSg) compound, HgBa.sub.2 Ca.sub.2 Cu.sub.3 O.sub.8+.delta. (Hg-1223) displays the highest transition temperature (T.sub.c) of 134 K at ambient pressure or 164 K at .about.30 GPa. It also exhibits a modest flux pinning strength which lies between those of YBa.sub.2 Cu.sub.3 O.sub.7 (YBCO) and Bi.sub.2 Sr.sub.2 Ca.sub.n-1 Cu.sub.n O.sub.2n+4+.delta. (BSCCO) [or Tl.sub.2 Ba.sub.2 Ca.sub.n-1 Cu.sub.n O.sub.2n+4+.delta. (TBCCO)] with n =1, 2, 3, . . . , suggesting a possible high critical current density (J.sub.c) at 77 K due to its higher Tc. Indeed, epitaxial thin films of HgBa.sub.2 CaCu.sub.2 O.sub.6+.delta. (Hg-1212), which have a T.sub.c .about.128 K and are homologous to Hg-1223, have been prepared and show a J.sub.c.about.10.sub.6 A/cm.sup.2 at 100 K, implying a J.sub.c.about.5.times.10.sup.6 A/cm.sup.2 at 77 K. Since Hg-1223 has a higher T.sub.c than Hg-1212, a J.sub.c.gtoreq.5.times.10.sup.6 A/cm.sup.2 at 77 K may be achievable in Hg-1223. Given the superior superconducting properties, Hg-1223 is therefore a desirable HTSg material for conductors, such as tapes and wires. Unfortunately, Hg-1223 is difficult to form and the compound is chemically unstable with the loss of Hg and the associated degradation at temperatures above .about.350.degree. C. This makes it extremely difficult to align the CuO.sub.2 -planes by the high-temperature melt-texturing technique commonly used for other HTSg materials. Added to the difficulty is the fact that the Hg released from the compound attacks many metals, especially at high temperatures, and forms amalgams.